This invention pertains to a power transmission for vehicles, such as, farm and agricultural tractors, and has three ranges of operation in both forward and reverse directions of vehicle travel derived from a single planetary differential. A low-speed range and a high-speed transport range are provided by a variable speed hydraulic transmission input to the differential and an intermediate speed work range is by hydromechanical operation with inputs to the differential from both the hydraulic transmission and a mechanical drive. More specifically, the hydraulic transmission embodies a variable speed reversible hydrostatic transmission with a variable reversible displacement unit, such as a pump, and a fixed displacement unit, such as a motor.
A two-mode power transmission with a low-speed hydraulic range and a higher speed hydromechanical range is known in the art. The prior application of this applicant identified above has added a high-speed transport range by an additional hydraulic range of operation. Said prior application also broadly shows the use of a reverse gear and clutch to provide three modes of operation in forward and reverse and a drop box configuration for a power transmission. The prior art and applicant's prior application do not have a three-speed range transmission operable in both forward and reverse, with the capability of gear interchange for variable high-speed range, possible utilization of multiple pumps and motors of the hydrostatic transmission by connection to both ends of shafts of the transmission and various other features embodied in this disclosure.